A cell system which includes a plurality of chargeable and dischargeable cell modules connected in parallel has been known. This cell system is adapted to charge electric power, from power sources that use natural energy, such as wind power generators and solar power generators, and electric power from commercial power sources, to the cell modules via a power conversion system such as an inverter and a converter.
Disclosed in PTL 1 is one example of a method for charging a plurality of cell modules connected in parallel, in which constant current control is started upon output of a specified constant current from a power source and, once the cell modules are sufficiently charged with the constant current and a current inflow to the cell modules decreases, the constant current charging is shifted to constant voltage charging.